Fullerenes are closed-cage molecules composed entirely of sp2-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) were first identified as closed spheroidal cages produced by condensation from vaporized carbon.
Fullerene tubes are produced in carbon deposits on the cathode in carbon arc methods of producing spheroidal fullerenes from vaporized carbon. Ebbesen et al. (Ebbesen 1), xe2x80x9cLarge-Scale Synthesis Of Carbon Nanotubes,xe2x80x9d Nature, Vol. 358, p. 220 (Jul. 16, 1992) and Ebbesen et al., (Ebbesen II), xe2x80x9cCarbon Nanotubes,xe2x80x9d Annual Review of Materials Science, Vol. 24, p. 235 (1994). Such tubes are referred to herein as carbon nanotubes. Many of the carbon nanotubes made by these processes were multi-wall nanotubes, i.e., the carbon nanotubes resembled concentric cylinders. Carbon nanotubes having up to seven walls have been described in the prior art. Ebbesen II; Iijima et al., xe2x80x9cHelical Microtubules Of Graphitic Carbon,xe2x80x9d Nature, Vol. 354, p. 56 (Nov. 7, 1991).
In defining carbon nanotubes, it is helpful to use a recognized system of nomenclature. In this application, the carbon nanotube nomenclature described by M. S. Dresselhaus, G. Dresselhaus, and P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Chap. 19, especially pp. 756-760, (1996), published by Academic Press, 525 B Street, Suite 1900, San Diego, Calif. 92101-4495 or 6277 Sea Harbor Drive, Orlando, Fla. 32877 (ISBN 0-12-221820-5), which is hereby incorporated by reference, will be used. The single wall tubular fullerenes are distinguished from each other by double index (n,m) where n and m are integers that describe how to cut a single strip of hexagonal xe2x80x9cchicken-wirexe2x80x9d graphite so that its edges join seamlessly when it is wrapped onto the surface of a cylinder. When the two indices are the same, m=n, the resultant tube is said to be of the xe2x80x9carm-chairxe2x80x9d (or n,n) type, since when the tube is cut perpendicular to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an arm chair repeated n times. Arm-chair tubes are a preferred form of single-wall carbon nanotubes since they are metallic, and have extremely high electrical conductivity. In addition, all single-wall nanotubes have extremely high thermal conductivity and tensile strength.
Single-wall carbon nanotubes (SWNT) have been made in a DC arc discharge apparatus of the type used in fullerene production by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus. See Iijima et al., xe2x80x9cSingle-Shell Carbon Nanotubes of 1 nm Diameter,xe2x80x9d Nature, Vol. 363, p.603 (1993); Bethune et al., xe2x80x9cCobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic Layer Walls,xe2x80x9d Nature, Vol. 63, p. 605 (1993); Ajayan et al., xe2x80x9cGrowth Morphologies During Cobalt Catalyzed Single-Shell Carbon Nanotube Synthesis,xe2x80x9d Chem. Phys. Lett., Vol. 215, p. 509 (1993); Zhou et al., xe2x80x9cSingle-Walled Carbon Nanotubes Growing Radially From YC2 Particles,xe2x80x9d Appl. Phys. Lett., Vol. 65, p.1593 (1994); Seraphin et al., xe2x80x9cSingle-Walled Tubes and Encapsulation of Nanocrystals Into Carbon Clusters,xe2x80x9d Electrochem. Soc., Vol. 142, p. 290 (1995); Saito et al., xe2x80x9cCarbon Nanocapsules Encaging Metals and Carbides,xe2x80x9d J. Phys. Chem. Solids, Vol. 54, p. 1849 (1993); Saito et al., xe2x80x9cExtrusion of Single-Wall Carbon Nanotubes Via Formation of Small Particles Condensed Near an Evaporation Source,xe2x80x9d Chem. Phys. Lett., Vol. 236, p. 419 (1995). It is also known that the use of mixtures of such transition metals can significantly enhance the yield of single-wall carbon nanotubes in the arc discharge apparatus. See Lambert et al., xe2x80x9cImproving Conditions Toward Isolating Single-Shell Carbon Nanotubes,xe2x80x9d Chem. Phys. Lett., Vol. 226, p. 364 (1994).
While this arc discharge process can produce single-wall nanotubes, the yield of nanotubes is low and the tubes exhibit significant variations in structure and size between individual tubes in the mixture. Individual carbon nanotubes are difficult to separate from the other reaction products and purify.
An improved method of producing single-wall nanotubes is described in U.S. patent application Ser. No. 08/687,665, entitled xe2x80x9cRopes of Single-Walled Carbon Nanotubesxe2x80x9d incorporated herein by reference in its entirety. This method uses, inter alia, laser vaporization of a graphite substrate doped with transition metal atoms, preferably nickel, cobalt, or a mixture thereof, to produce single-wall carbon nanotubes in yields of at least 50% of the condensed carbon. The single-wall nanotubes produced by this method tend to be formed in clusters, termed xe2x80x9cropes,xe2x80x9d of 10 to 1000 single-wall carbon nanotubes in parallel alignment, held together by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although one structure tends to predominate.
PCT/US/98/04513 entitled xe2x80x9cCarbon Fibers Formed From Single-Wall Carbon Nanotubesxe2x80x9d and which is incorporated by reference, in its entirety, discloses, inter alia, methods for producing single-wall carbon nanotubes, nanotube ropes, nanotube fibers, and nanotube devices. A method for making single-wall carbon nanotubes is disclosed, in which a laser beam vaporizes material from a target comprising, consisting essentially of, or consisting of a mixture of carbon and one or more Group VI or Group VIII transition metals. The vapor from the target forms carbon nanotubes that are predominantly single-wall carbon nanotubes, and of those, the (10, 10) tube is predominant. The method also produces significant amounts of single-wall carbon nanotubes that are arranged as ropes, i.e., the single-wall carbon nanotubes run parallel to each other. The laser vaporization method provides several advantages over the arc discharge method of making carbon nanotubes: laser vaporization allows much greater control over the conditions favoring growth of single-wall carbon nanotubes and the laser vaporization method produces single-wall carbon nanotubes in higher yield and of better quality. The laser vaporization method may also be used to produce longer carbon nanotubes and longer ropes.
PCT US99/25702 entitled xe2x80x9cGas-phase process for production of single-wall carbon nanotubes from high pressure COxe2x80x9d and which is incorporated by reference, in its entirety, discloses, inter alia, methods for producing single-wall carbon nanotubes, nanotube ropes, nanotube fibers, and nanotube devices. A method for making single-wall carbon nanotubes is therein disclosed, which invention comprises the process of supplying high pressure (e.g., 30 atmospheres) CO that has been preheated (e.g., to about 1000xc2x0 C.) and a catalyst precursor gas (e.g., Fe(CO)5) in CO that is kept below the catalyst precursor decomposition temperature to a mixing zone. In this mixing zone, the catalyst precursor is rapidly heated to a temperature that results in (1) precursor decomposition, (2) formation of active catalyst metal atom clusters of the appropriate size, and (3) favorable growth of SWNTs on the catalyst clusters. Preferably a catalyst cluster nucleation agency is employed to enable rapid reaction of the catalyst precursor gas to form many small, active catalyst particles instead of a few large, inactive ones. Such nucleation agencies can include auxiliary metal precursors that cluster more rapidly than the primary catalyst, or through provision of additional energy inputs (e.g., from a pulsed or CW laser) directed precisely at the region where cluster formation is desired. Under these conditions SWNTs nucleate and grow according to the Boudouard reaction. The SWNTs thus formed may be recovered directly or passed through a growth and annealing zone maintained at an elevated temperature (e.g., 1000xc2x0 C.) in which tubes may continue to grow and coalesce into ropes.
Carbon nanotubes, ropes of carbon nanotubes, and in particular, single-wall carbon nanotubes and ropes thereof, are useful for making electrical connectors in micro devices such as integrated circuits or in semiconductor chips used in computers because of the electrical conductivity and small size of the carbon nanotube. The carbon nanotubes are useful as antennas at optical frequencies, and as probes for scanning probe microscopy such as are used in scanning tunneling microscopes (STM) and atomic force microscopes (AFM). They are useful as electron field-emitters and as electrode materials, particularly in fuel cells andelectrochemical applications such as Lithium ion batteries. Carbon nanotubes may be used in place of or in conjunction with carbon black in tires for motor vehicles. The carbon nanotubes are also useful as supports for catalysts used in industrial and chemical processes such as hydrogenation, reforming and cracking catalysts. They are useful as elements of composite materials providing novel mechanical, electrical and thermal conductivity properties to those materials.
Ropes of single-wall carbon nanotubes are metallic, i.e., they will conduct electrical charges with a relatively low resistance. Ropes are useful in any application where an electrical conductor is needed, for example as an additive in electrically conductive paints or in polymer coatings or as the probing tip of an STM.
The present invention is directed to macroscopic materials and objects of aligned nanotubes and to the creation of such materials. The invention entails aligning single-wall carbon nanotube (SWNT) segments that are suspended in a fluid medium and then removing the aligned segments from suspension in a way that macroscopic, ordered assemblies of SWNT are formed.
The materials and objects are xe2x80x9cmacroscopicxe2x80x9d in that they are large enough to be seen without the aid of a microscope, or have the physical dimensions of such objects. These macroscopic, ordered SWNT materials and objects have the remarkable physical, electrical, and chemical properties that SWNT exhibit on the microscopic scale because they are comprised of nanotubes, each of which is aligned in the same direction and in contact or close proximity with its nearest neighbors. An ordered assembly of SWNT also serves as a template for growth of more and larger ordered assemblies. This invention shows the first realized means of creating macroscopic objects of aligned SWNT. These materials and objects are highly anisotropic: each of their physical properties such as electrical conductivity, thermal conductivity, tensile strength, compressive strength, resistance to fracture, etc. are dependent on the direction in which each of these properties is measured with respect to the direction of orientation of the SWNT in the object. The thermal conductivity, for instance, parallel to the direction of the SWNT in the object will be substantially different from the thermal conductivity in the direction perpendicular to the SWNT. In the following, any reference to physical properties is understood to refer to quantities that are anisotropic and appropriately described in the direction-dependent representations known to those skilled in the art.
According to one embodiment of the present invention, a single strand comprising millions of SWNT is disclosed. According to another embodiment of the present invention, a new material made of aligned single-wall carbon nanotubes is realized. This material is a thin (approx. 1.5 xcexcm thick) membrane having about 1014 individual nanotubes per cm2 oriented in the same direction, and lying in the plane of the membrane. This xe2x80x9cin-plane membranexe2x80x9d represents a new material, and is the first example of a macroscopic ordered assembly of carbon nanotubes.
According to another embodiment of the present invention, a method for alignment is disclosed. Electric fields, magnetic fields, and shear flow fields are known to apply forces to SWNT, and can be used to achieve alignment of SWNT segments suspended in liquids. One method involves applying the magnetic field to a suspension of SWNT segments, which are typically 200-1000 nanometers long. The interaction of the magnetic field with the SWNT segments causes a high degree of alignment of the individual segments in a direction parallel to the magnetic field. Once the segments are aligned, assembly of larger objects is achieved by enabling these aligned SWNT to come out of suspension and aggregate while they remain aligned by the magnetic field.
As the nanotube segments are removed from suspension, they adhere to one another in arrays wherein the tube segments lie essentially parallel to one another and each tube segment is in contact with its nearest neighbors. This proclivity of nanotube segments for self-assembly into small ordered structures has been known for several years. These small structures are often called xe2x80x9cropesxe2x80x9d, and typically have cross sections comprising between 10 and 1000 individual tubes. Ropes form naturally in all known production methods for SWNT and they appear in the solid residues from filtration or centrifugation of suspensions of SWNT segments in liquids.
This xe2x80x9cropingxe2x80x9d of SWNT happens in collisions and subsequent interaction of individual SWNT with one another, in interactions between individual SWNT and ropes that have already formed, or in interactions between ropes. xe2x80x9cRopingxe2x80x9d occurs because SWNT are exceedingly stiff molecules. The bare walls of SWNT have a strong van der Waals attraction for one another, and the tubes aggregate very easily. SWNT suspended in a liquid are mobile, and will move in a way consistent with well-known principles of physics. When two such stiff objects with attractive forces between their sides encounter one another, if they are free to rotate, they will reorient to the most energetically-favored arrangement, which is to lie together in such a way that there is a maximal contact surface area between the two entities. As long as the forces between the sides of the stiff objects are attractive, the condition of maximal surface contact is the condition of minimum energy for the system. Likewise, an individual tube segment aggregating with a xe2x80x9cropexe2x80x9d will align with the long axis of the rope, and lie so that it contacts two other tubes in the rope. When rope segments aggregate, they will rearrange themselves into a single rope of larger cross section, in a way that the energy of the structure so formed is minimized.
A further novel property of SWNT is that their surfaces are relatively smooth on an atomic scale, and there is little resistance to a motion in which one tube xe2x80x9cslidesxe2x80x9d in a direction parallel to its nearest neighbor. Thus as the rope forms, its constituent nanotube segments will further rearrange their displacements parallel to the axis of the forming rope in a way that minimizes energy. As indicated above, this minimization of energy occurs when the contact area between adjacent tubes is maximized, thus minimizing the exposed tube surface area. This principle dictates that as the rope forms, individual SWNT segments pack tightly, with the end of each segment in close proximity to the end of its nearest neighbor that lies along essentially the same axis. If the individual nanotube segments remain sufficiently mobile, as small ropes aggregate to form larger ropes, a similar repacking to minimize energy will take place, ensuring that the larger ropes are closely-packed with a minimum of voids inside.
Obviously, since xe2x80x9cropingxe2x80x9d depends on physical interactions of distinct SWNT or ropes, the rate of the xe2x80x9cropingxe2x80x9d process depends on the local concentration of nanotube material. If the local concentration of nanotube material is increased, roping proceeds more rapidly. The progress of the roping process, and the ultimate product can be controlled by modifying the environment of the nanotubes and the history of that environment prior to and during the roping process. An important aspect of this invention is to provide means of said modifications to exploit and control the xe2x80x9cropingxe2x80x9d behavior of SWNT to produce novel materials and objects comprising SWNT.
SWNT are highly anisotropic, and have remarkable physical properties. Likewise, a material that comprises highly-oriented SWNT, all arranged in the same direction will have remarkable properties. This invention presents the first realization of such material, means for producing such material and objects made from it, and several applications of this new composition of matter.
This invention first comprises the modification of the roping process by chemical means, wherein the diameter of the ropes formed is increased well beyond that known in the art. The larger ropes themselves form novel materials and objects.
If individual mobile tubes or rope segments are aligned by some means (electric, magnetic, shear, etc.), prior to or during the xe2x80x9cropingxe2x80x9d process they will form aligned ropes, which will then interact to form larger ropes. One aspect of this invention is to effect an alignment of the individual tube segments or small xe2x80x9cropesxe2x80x9d that enables their self-assembly on a larger scale that forms manipulable, macroscopic structures and materials. The invention also comprises the materials of highly-aligned SWNT segments and ropes of SWNT segments exemplified by the product of the demonstrated process, and it also includes electrical, chemical, mechanical, and biological applications of macroscopic ordered nanotube materials and objects.
The invention also comprises post-processing of said macroscopic ordered nanotube materials and objects in ways that are enabled by their ordered arrangement. This post-processing enhances the properties of said materials and objects by, for instance, by modifying their mechanical properties, electrical conductivity, thermal conductivity, and interaction with electromagnetic radiation. Such post-processing includes methods and techniques of joining the ends or sides of the nanotube segments that make up the macroscopic ordered material or object.
The ordered composition itself enables such post processing by maintaining the relative positions of interacting tube segments during the time required for interactions that comprise the post-process. The ordered assembly clearly holds the sides of individual nanotubes in contact with the sides of its nearest neighbors. Less obvious is that the efficient packing of nanotube segments as they aggregate during the formation process for the macroscopic ordered material or object also causes the ends of adjacent, co-linear nanotube segments to touch or be in very close proximity to one another. The ordered arrangement of the nanotube segments in the said material or object holds the adjacent nanotube segment ends in close proximity, enabling a post-processing step.
A simple example of such post processing is to introduce an agent or combination of agents that induce cross-linking between the sides of the tubes as they lie parallel to one another. Such agents include chemical ones that intercalate the ordered material and bond chemically to adjacent tubes, and a combination of radiation (photons, x-rays, gamma rays, and/or energetic ions, electrons or neutrons) and heat wherein the radiation causes dislocations in the regular tube sidewalls, and the heat enables rearrangements of the tube sidewalls in which bonds form between the wall of one tube segment and the wall of an adjacent tube segment. Said cross linking would materially alter the properties of the material or object by changing its shear strength, tensile strength, toughness, electrical conductivity and thermal conductivity.
Another example of such post-processing comprises application of heat, annealing of a macroscopic ordered nanotube material or object in such a way that the ends of essentially-collinearly-arranged and abutting nanotube segments rearrange their chemical bonding so that the segments become substantially joined by covalent bonds. This xe2x80x9cwelding togetherxe2x80x9d of individual nanotube segments at or near their ends within a macroscopic ordered material or object will alter and improve one or more components of the mechanical properties, electrical properties and thermal properties of the that material. For instance, the tensile strength, electrical conductivity and thermal conductivity of the material in the direction parallel to the tube axes are all increased by the xe2x80x9cwelding togetherxe2x80x9d of SWNT segments in the macroscopic ordered nanotube material. The ordered materials produced by this invention have a number of useful forms. They are presented as solid objects, films, and fibers. The unique xe2x80x9cropingxe2x80x9d behavior of SWNT and the control of that behavior disclosed herein also enable them to form sparse networks that are mechanically and electrically continuous. These networks, by themselves, or as elements of composite materials, enable creation of conductive polymers and films for management of electron flow in and around otherwise-electrically-insulating materials and structures.
It is a technical advantage of the present invention that a method for chemical manipulation of single-wall carbon nanotubes that enables production of large ropes and a macroscopic ordered assembly of carbon nanotubes is disclosed.
It is technical advantage of the present invention that a method for magnetic manipulation of single-wall carbon nanotubes and ropes is disclosed
It is a technical advantage of the present invention that methods for producing a macroscopic ordered assembly of carbon nanotubes are disclosed. It is a technical advantage of the present invention that methods for post processing a macroscopic ordered assembly of carbon nanotubes that modify the properties of said assembly and are fundamentally enabled by the assembly""s structure are disclosed.
The foregoing objectives, the compositions of matter produced by them and other objectives apparent to those skilled in the art, are achieved according to the present invention as described and claimed herein.
The ordered assemblies also are important in their service as a substrate for initiation of growth of more and larger ordered assemblies of nanotubes. Here, the ordered assembly is cut in a direction perpendicular to that of the tube axes. The exposed surface is then cleaned and made uniform using electrochemical polishing or other means known to those skilled in the art of surface science. A transition metal catalyst is placed on or near the open tube ends., The catalyst is either in the form of metal deposited by a known means or pre-formed metal clusters with attached chemical moieties that enable of the clusters to communicate with and join with the open tube ends. This assembly is then exposed to a growth environment. One such environment is 30 atmospheres of CO at a temperature of approximately 1000xc2x0 C. The catalyst metal becomes mobile at elevated temperatures and forms small clusters on the open tube ends, and the individual tubes begin growing in an ordered array of the same tube type, diameter, and spacing as the original substrate array. This process enables assembly of fibers, cables, and structural materials that will be more than an order of magnitude stronger than any others that can now be produced. The materials may be used to produce structural sections such as I-beams, composite structures, electrodes, structural and/or active parts of batteries, armor and other protective materials, thermal management structures or devices, and structures or devices that reflect, absorb or modify electromagnetic radiation impinging upon them.
The methods of the present invention are fundamentally enabling in both the assembly of xe2x80x9cseed arraysxe2x80x9d for further nanotube growth, particularly for growth of additional macroscopic, ordered nanotube materials and structures.